This invention relates, in general, to a process for refining steel, and more specifically, to an improvement in the basic oxygen process wherein molten steel contained in a vessel is refined by top blowing oxygen into the melt, i.e., from above the melt surface.
The manufacture of steel by the basic oxygen process, also referred to as BOP or BOF process, is well known in the art. When low carbon steel is made by this process, it is often subject to contamination by atmospheric nitrogen. Such contamination tends to cause premature age hardening of the steel, which leads to strain-aging, poor surface properties and undesirable appearance of the final product.
The problem of nitrogen pickup during the manufacture of low-carbon steels has been addressed by the prior art. Glassman, in U.S. Pat. No. 3,769,000, describes a method for excluding nitrogen from the melt by placing a hood loosely over the mouth of the refining vessel. Nitrogen from ambient air is excluded by maintaining a curtain of carbon dioxide between the hood and the refining vessel. Pihlblad et al., in U.S. Pat. No. 3,307,937 disclose a method for excluding atmospheric nitrogen from the melt by adjusting the size of the opening through which gas flows out at the top of the vessel, thereby maintaining positive pressure in the vessel with respect to the ambient atmosphere, even at low carbon levels. Both of these approaches require modification of the BOP vessel which is expensive and cumbersome to utilize; consequently, neither has met with significant commercial success.
In addition to the potential for nitrogen contamination, a second disadvantage of the conventional basic oxygen process is the increasing quantity of oxygen that reacts with valuable metal as the carbon content of the melt decreases. Several U.S. patents disclose ways of diluting the oxygen with another gas in order to minimize the amount of oxygen that reacts with the metal. Such patents include Fulton et al. U.S. Pat. No. 3,649,246 and Ramachandran's U.S. Pat. Nos. 3,594,155 and 3,666,439. These patents deal only with the problem of increasing the degree to which the injected oxygen reacts with carbon rather than the metal. None are concerned with how one might utilize a diluent to minimize nitrogen pickup from the atmosphere during oxygen decarburization in the BOF.